You may be correct about the pipe material in Illinois. It’s been a long time since I was licensed there but the discharge pipe must be metal (this is going back 15 years ago). There was a time when a piece of 3/4” gas pipe for the discharge pipe was all good. None of this noise about having to be of the same material as the potable water piping. It may or may not be that way today. Copper prices drove Illinois to adopt CPVC pipe for potable water distribution years ago. The take away is that every jurisdiction is different.
In the past I have spoken about this with my plumber, and had the same info referenced at several CE classes. Not more than one 90 degree bend and ONLY if the drain line is associated with a valve ON TOP OD the water heater. Need a “gap”. If in a finished location should “exhaust” into a pan which then drains to wherever. Pipe end NOT threaded. End of pipe 4 - 6 inches from floor. Pipe threaded into the valve. SAME diameter as the valve, NO reduction in pipe diameter. CPVC only if supplied by manufacturer, otherwise copper. NO opening below the floor, typically into the crawlspace or basement ceiling insulation. MUST be visible to see if valve is dripping, a precursor of potential failure and indicator of the need for a qualified plumber.
Codes are difficult to blanket quote as every jurisdiction is different. This is from the 2018 Uniform Plumbing Code still used by some states.
See 608.5 (7)
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Below is the 2018 IRC requirements. Be aware this has changed over the years and some cities or states may have additional requirements or made modifications to the original code.
IRC 2018
P2804.6.1 Requirements for discharge pipe. The discharge piping serving a pressure relief valve, temperature relief valve or combination valve shall:
- Not be directly connected to the drainage system.
- Discharge through an air gap located in the same room as the water heater.
- Not be smaller than the diameter of the outlet of the valve served and shall discharge full size to the air gap .
- Serve a single relief device and shall not connect to piping serving any other relief device or equipment.
- Discharge to the floor, to the pan serving the water heater or storage tank, to a waste receptor or to the outdoors.
- Discharge in a manner that does not cause personal injury or structural damage.
- Discharge to a termination point that is readily observable by the building occupants.
- Not be trapped.
- Be installed to flow by gravity.
- Terminate not more than 6 inches (152 mm) and not less than two times the discharge pipe diameter above the floor or waste receptor flood level rim.
- Not have a threaded connection at the end of the piping.
- Not have valves or tee fittings.
- Be constructed of those materials indicated in Section A112.4.1.
- Be one nominal size larger than the size of the relief-valve outlet, where the relief-valve discharge
piping is installed with insert fittings. The outlet end of such tubing shall be fastened in place.
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Yes correct very similar to the uniform plumbing code with the exception of discharging the TPR valve into the pan and some other variations. The reason for the post is to just bring awareness that if we as home inspectors are going to call out a defect we need to be aware of the local codes or contact a local plumber for verification.
I don’t call it code. I call it “common cents personal safety”
Tsk, tsk, tsk! No good!
Now you need to prove to your client there are too many elbows. Where is that information located?
In the installation manual of the TPRV, otherwise within reason 
Your cheating the question was for the OP to figure out. You’re close but not the answer I’m looking for.
Ops… 
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That is creative! Double air gap?
Just to be clear, I just came across an installation that only had two 90 degree bends, however terminated into a “French drain”. I thought the discharge had to be a certain distance above the floor to prevent excessive splashing of hot water if a discharge were to occur.
Dave, just to be clear, did the TPRV drain go to the exterior and terminate into a “French Drain” system? Was the termination end visible?
Installed wrong.
TPR Valves and Discharge Piping
Dave, just to clarify even more, I need to ask: Did you mean “Floor drain” , or do you really mean “French Drain” meaning the TPR discharge pipe went outside the home and down into a perimeter drain one or two feet underground at the foundation exterior?
This is a “French Drain”
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It’s hard to tell from the pic, are those horizontal lines sloped at all? They look fairly level.
(IRC 2018 P2804.6.1 Requirements for discharge pipe. The discharge piping serving a pressure relief valve, temperature relief valve or combination valve shall: 9. Be installed to flow by gravity.)
Just a thought I had. I’m probably wrong. 
The TPRV manufacture recommends associated number of elbows and pipe feet length rules.
I think Cash Acme is 2 elbows 15’ foot run. Watts being 3 elbows and a 30’ foot run.
If that is a Watts, more than 4 elbows will cause a reduction in discharge capacity. in other words, slow down the rate of flow but the water will discharge. Discharge to the receptor is fine.
Yes it was visible. Did not go through the wall, but the end was about 1/2” above the ground. Ran down the heater, across the floor and onto the drain along the wall.